How due dates are estimated
The standard method — Naegele's rule, used by doctors worldwide — counts 280 days (40 weeks) from the first day of the last menstrual period. It assumes a 28-day cycle with ovulation around day 14, so this calculator adjusts when your cycle runs longer or shorter: a 35-day cycle typically shifts ovulation, and therefore the due date, about a week later.
How to use it
- Enter the first day of the last period — the first day of bleeding, not the last.
- Select your average cycle length (28 is average; anywhere from 21 to 35 is common).
- Read the estimated due date, today's week of pregnancy, the current trimester, and the milestone table — including when the third trimester begins and the full-term window.
Keeping the estimate in perspective
- The "due date" is a midpoint, not an appointment. Only about 1 in 20 babies arrives on the exact date; the great majority come within two weeks either side. "Due month" is the healthier way to think about it.
- Early ultrasound refines the date. A first-trimester scan measures the baby directly and is the most accurate dating method — if the scan date differs from the LMP date, clinicians go with the scan.
- Irregular cycles make LMP dating less reliable — mention it at the first antenatal visit so dating can lean on ultrasound.
- Tracking dates in the Nepali calendar? The Nepali Date Converter moves any date between BS and AD.
This calculator is general information for planning and curiosity — antenatal care decisions belong with your doctor or midwife.
Frequently asked questions
How accurate is a due date from LMP?
It's a well-established estimate, not a promise — only about 5% of babies arrive on the exact date, and most come within two weeks either side. A first-trimester ultrasound refines the estimate and takes precedence when the two differ.
My cycles aren't 28 days — does that matter?
Yes, and the calculator handles it: pick your average cycle length and the due date shifts accordingly (about one day per day of cycle difference). Very irregular cycles make LMP dating unreliable — mention it at your first antenatal visit.
When do the trimesters start?
Using the standard convention: the second trimester begins at 14 weeks 0 days, the third at 28 weeks 0 days, and full term at 39 weeks. The milestone table shows your personal calendar dates for each.
Is anything I enter stored?
No. Like every tool on this site, the calculation happens in your browser and disappears when you close the page — appropriate for information this personal.